Exercise devices, and in particular weight training machines, typically include a mechanical member that the user repeatedly moves along a prescribed path for exercise. Conventionally, movement of the mechanical member is resisted in some fashion (often by weights) to render the movement more difficult and thereby intensify the exercise. The movement of the mechanical member determines what muscle or muscle groups are to be involved in the exercise.
One popular exercise movement is the leg press, which involves straightening the legs from a position in which they are bent at approximately a ninety degree angle at the knees and in which the exerciser's thighs are bent so that the exerciser's knees are drawn somewhat near the chest. Generally, the leg press movement exercises the quadriceps, gluteals, and hamstrings of the exerciser.
The leg press movement can be carried with different types of apparatus. For example, some machines are configured such that the exerciser lays on his back with the thighs generally upright and applies a generally horizontal force against a foot panel with the heels. Such machines can have a “sled” configuration, in which a platform supporting the exerciser slides rearwardly as he pushes his feet against a foot panel, or a configuration in which the foot panel moves relative to the frame in response to the exerciser pushing against it. Such a machine is exemplified by the NITRO™ Leg Press, available from Nautilus HPS, Inc., Independence, Va. As another example, machines exist in which the exerciser is in a generally upright or inclined seated position with his back placed against a backrest and applies a generally horizontal force through the heels against a foot panel. These machines can also have a “sled”-type design, in which the seat and backrest slide relative to the frame, or a stationary seat design, in which the foot panel moves relative to the frame. An exemplary machine of this type is the 2ST leg press machine, available from Nautilus HPS, Inc., Independence, Va. (also illustrated in U.S. Pat. No. 5,106,081).
One issue that can arise with leg press machines is the angle at which force is applied to the foot panel. Ideally, this force should be applied parallel to the tibia of the user (i.e., the foot panel is normal to the user's tibia) to reduce shear stress on the knee joint and to increase exercise efficiency. With a sled-type design, typically the foot panel is stationary, so the angle of the tibia to the foot panel varies as the leg straightens. With a typical simple swing arm-type design in which the foot pad moves relative to the frame, the foot panel tends to increase its angle relative to the exerciser's back as the foot panel moves away from the user. Unfortunately, this variation in angle should decrease in order to maintain the desired ninety degree angle with the tibia. One approach to address this problem is illustrated in the Nautilus 2ST leg press machine discussed above, in which a pair of swing arms interconnect with a base and a member connected to the foot pad to decrease the angle of the foot pad in the desired manner.
Another potential issue with leg press machines is the degree of resistance experienced by the user during the exercise stroke. It is well-known in this art that the leverage and mechanical advantage enjoyed by the exerciser during typical weight training vary at different points along the path defined by an exercise movement. In order to increase the efficiency of the exercise, exercise machine designers often design machines so that the resistance experienced by the exerciser varies within the exercise stroke to match the extent of the exerciser's mechanical advantage. The variation in resistance is typically carried out with cams and/or mechanical linkages that couple the exercise movement member and the resistance. In the leg press movement, the exerciser has his least mechanical advantage when his thighs form a ninety degree angle with his lower legs, and has a much greater mechanical advantage as the legs become more completely extended. Consequently, leg press machines are often designed so that the resistance at the end of the press movement is much higher than that at the beginning of the movement. Although the resistance variation for leg press machines has been satisfactorily accomplished with cams (which tend to be more expensive to produce than mechanical linkages), to date the mechanical linkages designed to vary resistance in leg press machines have generally had some shortcomings, particularly for leg press machines that attempt to provide and maintain the ideal ninety degree relationship between the exerciser's tibia and the foot panel.